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Re: Re(2) (ichimunki): Security issues when allowing file upload via CGI

by nufsaid (Beadle)
on Dec 07, 2001 at 00:42 UTC ( #130061=note: print w/ replies, xml ) Need Help??


in reply to Re(2) (ichimunki): Security issues when allowing file upload via CGI
in thread Security issues when allowing file upload via CGI

I like the idea, but don't you have to be careful of this line?

my $file_cmd_output = `file $fn`;

$fn is tainted and doesn't this give them the chance to sneak a command in via $fn? Need to make sure $fn is clean.

Joe.


Comment on Re: Re(2) (ichimunki): Security issues when allowing file upload via CGI
Re(4) (ichimunki): Security issues when allowing file upload via CGI
by dmmiller2k (Chaplain) on Dec 07, 2001 at 01:50 UTC

    Sigh.

    Yes, you're right about $fn being tainted, but I was speaking more to the principle of using the standard *NIX command, "file" (or reasonable facsimile, such as the perl module File::MMagic, for example) to check the type of the file by examining the first several bytes (or more).

    Certainly, by the time I intended this code to run, a file would have been uploaded to some directory somewhere. My expectation was that $fn would, at the time of the call, contain the full pathname to this file, and would not necessarily have been entered directly by a web-page user.

    But then, I DID fail to document these expectations (read: preconditions, using programming-by-contract terminology), so I deserve to be chastised for it.

    (takes 40 lashes with a wet noodle)

    dmm

    
    You can give a man a fish and feed him for a day ...
    Or, you can teach him to fish and feed him for a lifetime
    

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